The present invention relates to a circuit that drives a group of driven elements such as light-emitting diodes, heating elements, or display elements, and to a printer employing this driving circuit.
The invented driving circuit can be fabricated as an integrated circuit (IC) incorporating complementary metal-oxide-semiconductor (CMOS) logic. One object of the invention is to simplify the testing of an integrated driving circuit of this type.
A simple method of testing a CMOS IC is to measure the amount of current drawn by the IC in its quiescent state, when all inputs to the IC are held at constant logic levels. In principle, a CMOS IC draws current only when its input levels change; no current should flow in the quiescent state. In practice, if the IC is free of electrical faults, the measured quiescent current (IDDq) is in the picoampere range, or is at most a few microamperes. Internal defects that cause short circuits between adjacent signal paths in the IC are revealed when a higher quiescent current is measured.
This test, known as a quiescent-current test or IDDq test, detects electrical faults more quickly and efficiently than conventional functional tests, which apply numerous test patterns to the input terminals of the IC and check the resulting outputs at the output terminals to confirm that internal logic functions are being performed correctly.
The efficient and inexpensive IDDq test cannot be applied to many conventional driving ICs, however, because these ICs are not purely CMOS circuits. They include amplifiers with internal current sources that draw current at all times, even in the quiescent state. The current drawn by these current sources is so large that any variations in quiescent current caused by electrical faults in logic circuits are effectively masked. Less efficient and more expensive functional tests must therefore be carried out instead.
When this type of conventional driving IC is employed in a printer, a further problem is that the current sources draw current whenever the printer""s power is switched on, even if the printer is not operating. Since a printer may include many driving ICs, the current drawn can significantly increase the printer""s standby power consumption, raising environmental issues. Under the International Energy-Star program, for example, the standby power consumption of a printer or facsimile machine in the maximum seven-page-per-minute class is only fifteen watts (15 W). Another object of the invention, accordingly, is to enable a printer to meet stringent standby power-consumption requirements.
The invented driving circuit drives a plurality of driven elements according to externally supplied driving data. The driving circuit includes a plurality of drive output circuits, and a transfer circuit that transfers the driving data to the drive output circuits. The drive output circuits supply driving current to the driven elements, responsive to the driving data and a control voltage.
The control voltage is generated in a control-voltage generating circuit having a static current path. The control-voltage generating circuit also has a switching element that opens and closes the static current path. The switching element is controlled by a control circuit that causes the switching element to open the static current path when a standby command is received. The control-voltage generating circuit and the control circuit are part of the driving circuit.
The standby command may be given by a single external signal. Alternatively, the standby command may be given by a combination of external signals that are also used for controlling other circuits, such as the transfer circuit and drive output circuits.
The invented driving circuit may be an integrated circuit formed on a single semiconductor chip. The invention also provides a printer having a plurality of such integrated driving circuits. The printer includes a printing control unit that gives the standby command to the driving circuits.
By opening the static current path, the invented driving circuit and printer reduce standby power consumption. The standby command also reduces current consumption to a level low enough to permit quiescent-current testing of the invented driving circuit.